session: force tidb_multi_statement_mode=OFF when mode=WARN (#25723)

[ti-srebot]

cherry-pick #25723 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25751

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/25751:release-5.1-1cab530b793a

---

What problem does this PR solve?

Problem Summary:

In TiDB 5.0 the default for tidb_multi_statement_mode=OFF, but for upgrades the previous default from 4.0 could be inherited. This changes the value for upgrades as well.

From the docs, "Only the default value of OFF can be considered safe". This was always known to be the case, WARN was introduced to provide a nagging upgrade path for users. Now its time.

What is changed and how it works?

What's Changed:

For users upgrading from TiDB 4.0, the value of tidb_multi_statement_mode is now OFF. It is recommended to use the multi-statement feature of your client library instead, see the documentation on tidb_multi_statement_mode for additional details.

Related changes

• Need to cherry-pick to the release branch
• The cherry pick to 4.0 will look a bit different as it also needs to change the sysvar default.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Bootstrap tasks are hard to test with automated testing. I launched a 4.0.13 server and then ran the SQL statement in this bootstrap task and confirmed only 1 row was modified, and the value changed to OFF.

Side effects

• Breaking backward compatibility

Release note

• For users upgrading from TiDB 4.0, the value of tidb_multi_statement_mode is now OFF. It is recommended to use the multi-statement feature of your client library instead, see the documentation on tidb_multi_statement_mode for additional details.

[ti-srebot]

/run-all-tests

[ti-srebot]

@morgo please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/tidb/invitations

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• bb7133
• xhebox

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[bb7133]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: a4708b9

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[morgo]

/run-unit-test